A typical directional-control valve is comprised of a valve spool that slides linearly within a valve body. The valve spool is comprised of a discrete number of lobes, typically three, while the valve body is comprised of a discrete number of internal and external ports, typically five. As the valve spool slides linearly through the valve body, various ports in the valve body are covered or exposed by the lobes of the valve spool, which in effect provides various configurations of connectivity between the ports in the valve body. The valve spool of a typical directional-control valve incorporates three lobes, while the valve body of a typical directional-control valve incorporates five ports, which correspond to four distinct fluid ports, which are the supply (S), exhaust (E), and first and second actuator ports (A and B, respectively). When the valve spool is in the first position (P1), the inlet port (S) is in fluid communication with the second actuator port (B), and the exhaust port (E) is in fluid communication with the first actuator port (A). When the valve spool slides to the second position (P2), the inlet port (S) is in fluid communication with the first actuator port (A), and the exhaust port (E) is in fluid communication with the second actuator port (B). These are the two fundamental positions of a directional-control valve, and valves that provide these two positions (P1 and P2) exclusively are called two-position directional-control valves.
In a two-position valve, the valve spool is commonly (though not universally) moved within the valve body between the first and second spool positions by a single solenoid actuator. The solenoid actuation can either be configured to be a direct-acting type, in which a solenoid actuator exerts a motive force directly on the valve spool, or of the pilot-actuated type, in which the solenoid actuator controls a pilot valve, which in turn controls the flow of a pressurized fluid, which in turn exerts motive force on the valve spool. The latter is in effect a mechanism of force amplification.
In the case of a two-position valve, the valve is often actuated by a single solenoid actuator. In the direct-acting type, energizing the solenoid actuator pushes the spool directly into the first position, while de-energizing the solenoid allows a return spring (or a similar return mechanism) to push the spool back to the second spool position. In the pilot-operated type, energizing the solenoid typically opens a pilot valve, which in turn enables a supply of pressurized fluid to fill a small cylinder adjacent to the spool and push the spool into the first position. De-energizing the solenoid de-pressurizes the pilot cylinder and allows a return spring, a source of pressurized fluid, or a similar return mechanism, to push the spool back to the second spool position.
In some embodiments of a directional-control valve, the valve includes a third position of the spool within the valve body, which is physically located in an intermediate position approximately halfway between the first and second positions of the spool relative to the valve body. The third position is characterized by a different type of fluid connectivity relative to the first and second positions. For example, in the third position, all inlet and actuator ports might be isolated (i.e., none of the four ports are in fluid communication with each other), or both actuator ports might be connected to an exhaust port. Valves that provide the two fundamental positions, in addition to a third intermediate position, are called three-position directional-control valves.
In the case of a typical three-position valve, the valve is actuated by a pair of opposing solenoid actuators, such that energizing one solenoid while de-energizing the other moves the spool into the first position; reversing this pattern moves the spool into the second position; and de-energizing both solenoids allows a pair of centering springs to move the spool to the third, intermediate position. A pilot-operated valve operates similarly.